Multi-port LAN switch for a token-ring network

ABSTRACT

A multi-port LAN switch is provided that enables attached network devices to both communicate directly and to insert into a token-ring network through an attached concentrator. Each port of the multi-port LAN switch has a first transformer, a second transformer, and a switching system. The first transformer has a first winding connected to a first connection of the port. The second transformer has a first winding connected to a second connection of the port. The switching system switches to a port mode or an adapter mode as a function of a mode signal transmitted to the port. In the adapter mode, the switching system connects a second winding of the first transformer to a transmitter circuit in the port, and its connects a second winding of the second transformer to a receiver circuit in the port. In the port mode, the switching system connects the second winding of the first transformer to the receiver circuit in the port, and it connects the second winding of the second transformer to the transmitter circuit in the port. Each port of the multi-port LAN switch further includes a DC voltage source that is switchably connected to the first connection of the port through a first switch. Each port further includes a second switch that connects the second connection of the port to a DC return-path in a first position, and connects the second connection to the first connection when in a second position.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates in general to an improved datacommunications networking system, and in particular to an improved localarea network (LAN) switch. Still more particularly, the presentinvention relates to a multi-port LAN switch for providing acommunications link between a network adapter and a networkconcentrator, a network adapter and another network adapter or a networkconcentrator and another network concentrator.

2. Description of the Related Art

In digital data transmission systems, composite clock and data signalsin binary form are transmitted over media such as wires or fiber opticcables from a transmission line transmitter to a transmission linereceiver. The transmitter and the receiver in a data communicationssystem may each be a single computer or may each comprise a local areanetwork (LAN) of computers. An individual computer or station in a LANmay both send information to other stations in the LAN and receiveinformation from other stations. The station inserts into the LAN whenit desires to communicate with another station in the LAN, and detachesfrom the LAN when the communications are complete.

A common LAN topology is the "token-ring" network. The token-ring isused to interconnect the devices attached to the network. The token-ringnetwork allows unidirectional data transmission between stations in aring-like circuit by a token passing procedure. The ring topologypermits tokens to be passed from a node associated with a particularattached device, such as a personal computer, to another node in thering. A node that is ready to send data can capture the token andthereafter insert data for transmission. If information received by anode or station is destined for a station further along the network, thereceiving station must pass the information along the LAN to the nextadjacent station, and so forth, until the information reaches its finaldestination. A device or computer station attempting to gain access to anode of the token-ring will have an adapter, which is physicallyconnected to the token-ring. This accessing device must carry out aprocedure following a standard protocol in order to access thetoken-ring.

One type of token-ting product has two data transmission speeds, 4 Mbpsand 16 Mbps. Both of the transfer speeds are frequently used, and often,the data transmission speed of 4 Mbps may be used in one network, whilethe data transmission speed of 16 Mbps may be used in another network,both of which a user may wish to access.

Many LANs employ concentrators, or hubs, also known as multi-stationaccess units, to connect many stations at a single network node. Thesemultistation access units connect individually with each station along a4-wire cable called a lobe. Multiple lobes extend out from aconcentrator to individual stations to form a star-like structure.Physically, each station is individually attached to the concentratorthrough its lobe where it may access the network node. All stationsattached to a particular concentrator operate at the same network speed(e.g., 4-Mbps). When the concentrator is connected to a token-ringnetwork, the logical configuration of the network places each stationconnected to the concentrator at a separate node within the ring. Aconcentrator can individually connect the attached devices in atoken-ring, or it may be connected with other concentrators to form alarger token-ring comprised of all the devices attached to allconcentrators. An intelligent concentrator is one that includesprocessor controlled switching electronics for controlling access to thenetwork.

A concentrator is usually referred to as a "Multi-Station Access Unit"or MAU. Such system is provided for in the IEEE 802.5 specification,which refers to such system as a "Trunk Coupling Unit." Single lobescomprised of two twisted-pair wires connect a network adapter or othercommunication device to a port of the concentrator. Single lobes arecombined with other identical lobes to form a complete concentrator.While the number of lobes in a concentrator can vary, the most popularconfiguration utilizes eight lobes and such is due primarily to thephysical size of the token-ring connector as it fits in a standardequipment rack.

The function of a MAU is to electrically insert and remove a workstationor personal computer from a communication networking system, or morespecifically, to connect or remove a workstation or personal computerfrom a token-ring network. Control of the insertion or removal of aworkstation from a token-ring network is accomplished, as specified inthe IEEE standards, by means of a DC voltage that is sometimes referredto as a "phantom drive current." This phantom drive is applied betweenthe two pairs of conductors in the data cable or lobe that connects theworkstation to the Multi-Station Access Unit. When the phantom drivecurrent (or voltage) is present at a preselected level or potential, theMAU functions to insert the workstation into the network. When thephantom drive is absent or falls below a preselected level, theworkstation is removed from the network. For a more detailed referenceto information relating to the operation of a MAU, reference may be madeto IEEE 802.5. In addition, a full-duplex (FDX) adapter can gain accessto the network by sending an FDX registration frame to a port on a LANswitch. The FDX frame is a special frame identifying an adapter orswitch port as having FDX capability. If an adapter or switch portreceives this frame, it responds by transmitting its own FDXregistration frame. After this FDX frame handshaking occurs, the phantomdrive is asserted. This method of insertion is described in U.S. patentapplication Ser. No.08/399,267, filed Mar. 6, 1995, entitled "Apparatusand Method for Determining Operational Mode for a Station Entering aNetwork.", Chorpenning, J., et. al. now U.S. Pat. No. 5,561,666,incorporated herein by reference.

Each computer attached to the network is connected to a respective lobeport of the concentrator via a cable, and the computer exercises controlof the insertion/bypass mechanism via the cable using the phantom drive.This DC voltage is transparent to the passage of computer-transmitteddata, hence the name "phantom". The impressed voltage is used within thelobe port of the concentrator to affect the serial insertion of thecomputer in the ring. Cessation of the phantom drive causes ade-insertion action that will bypass the computer and cause the computerto be put in a looped (wrapped) state.

A computer attached to the network contains a network adapter cardhaving the electronics and hardware necessary to both connect with a MAUvia a lobe and to insert and communicate in a token-ring network. Inexisting token-ring adapters, convention requires that data betransmitted on the orange/black pair of wires and received on thered/green pair of wires in the medium interface cable. Token-ringadapters connect directly to a MAU, such as the IBM® 8228, so that theMAU receives data on the orange/black pair of wires and transmits on thered/green pair of wires. The phantom drive current is asserted by thetoken-ring adapter on the orange/black pair of wires to allow insertioninto the token-ting. The phantom drive current provides a dual functionof both detecting faulty wiring and engaging the relay in the MAU toserially connect the computer into the token-ting. Thus, by convention,adapters source phantom drive and a MAU sinks phantom drive.

While the above described token-ring network allows every networkadapter to communicate with every other network adapter, thiscommunication must be performed over the token-ring network through aconcentrator. Consequently, two computers situated adjacent to eachother within the LAN must communicate using the network's limitedbandwidth, which is shared with every adapter attached to the network.There are two problems that prevent direct connection of two networkadapters. First, the direct connection of two network adapters meetingthe standard set forth in IEEE 802.5 would have their transmittwisted-pair (orange/black pair) directly connected, and their receivewires (red/green twisted-pair) directly connected, preventing anycommunication between the two network adapters. Second, currentlyavailable network adapters cannot sink phantom drive; they only sourcephantom drive. Without the capability to sink phantom drive, an adapterattempting to communicate would source phantom drive to the receivingadapter, which could not sink the phantom drive. Thus, the sourcingadapter would detect a "wire fault" condition. Upon detecting a wirefault condition, the adapter would automatically stop transmitting data.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide anapparatus that "crosses-over" the transmit and receive wires of networkadapters attached to the apparatus to allow direct communication betweenthe adapters. It is a further object of the present invention to providea phantom drive sink capability to prevent an erroneous "wire fault"condition from being detected when two network devices are in directcommunication. It is a still further object of the present invention toprovide a direct communication link between two network devices thatprovides the full network bandwidth (for example, 16 Mbps), rather thanthe shared bandwidth provided by a connection through the token-ringnetwork.

According to the present invention, a multi-port LAN switch is providedthat enables attached network adapters to both communicate directly andto insert into a token-ring network through an attached concentrator.Each port of the multi-port LAN switch has a first transformer, a secondtransformer, and a switching system. The first transformer has a firstwinding connected to a first connection of the port. The secondtransformer has a first winding connected to a second connection of theport. The switching system switches to a port mode or an adapter mode asa function of a mode signal. In the adapter mode, the switching systemconnects a second winding of the first transformer to a transmittercircuit in the port, and it connects a second winding of the secondtransformer to a receiver circuit in the port. In the port mode, theswitching system connects the second winding of the first transformer tothe receiver circuit in the port, and it connects the second winding ofthe second transformer to the transmitter circuit in the port.

According to another feature of the present invention, each port of themulti-port LAN switch further includes a DC voltage source that isswitchably connected to the first connection of the port through a firstswitch. Each port further includes a second switch that connects thesecond connection of the port to a DC return-path in a first position,and connects the second connection to the first connection in a secondposition.

The above as well as additional objects, features, and advantages of thepresent invention will become apparent in the following detailed writtendescription.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the invention are setforth in the appended claims. The invention itself however, as well as apreferred mode of use, further objects and advantages thereof, will bestbe understood by reference to the following detailed description of anillustrative embodiment when read in conjunction with the accompanyingdrawings, wherein:

FIG. 1 shows a data communication and networking system according to apreferred embodiment of the present invention.

FIG. 2 shows a schematic diagram of a single port of the multi-port LANswitch of the present invention in an adapter mode, in accordance withthe preferred embodiment thereof.

FIG. 3 depicts a single port of the multi-port LAN switch of the presentinvention configured in a port mode, in accordance with the preferredembodiment of the present invention.

FIG. 4 shows a standard adapter connected to a port of the LAN switch ofthe present invention configured in a port mode in accordance with thepreferred embodiment of the present invention.

FIG. 5 depicts a port of the multi-port LAN switch of the presentinvention configured in adapter mode for connection with the standardMAU, in accordance with the preferred embodiment of the presentinvention.

FIG. 6 depicts a schematic block diagram showing a port of a first LANswitch in the adapter mode and a port of a second LAN switch in the portmode, in accordance with the preferred embodiment of the presentinvention.

FIG. 7 depicts a flow diagram of the preferred method of automaticallydetermining an attached device type and configuring the multi-port LANswitch of the present invention.

FIG. 8 depicts a flow diagram continuing the flow diagram of FIG. 7showing the preferred method of automatically determining the connectiontype of an attached device and configuring the multi-port LAN switch ofthe present invention, in accordance with a preferred embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference now to the figures and in particular with reference toFIG. 1, there is depicted a high-level block diagram of a communicationsnetworking system, which includes a multi-port LAN switch in accordancewith the present invention. FIG. 1 shows a digital data communicationnetworking system 10 into which is incorporated the multi-port LANswitches (40, 46, 47) of the present invention. The system 10 comprisesa local area network (LAN), which is formed by a plurality of attacheddevices or stations 12, such as personal computers or workstations. Thestations 12 are connected to each other by concentrators 14 and 44 andby LAN switches 40, 46, 47. A typical concentrator can receive up to 8attached devices, and can complete the network connection between theattached devices standing alone. A concentrator may also be attached toa main network connecting multiple concentrators to form a largernetwork over a greater geographic area.

Each station 12 is connected to concentrators 14, 44, such as the IBM®8228 MAU, at an attachment port 16 by a lobe, such as lobe 22, which isa transmission media. The particular attached device or station 12 isinserted into the network by means of a network adapter (24, 36, 37, 42,43) contained in the station 12. The adapter provides the directconnection to the lobe and the mechanism by which each of the stationsgain access to the network to send and receive data, and it contains thehardware and/or software necessary to physically connect with andoperate within the network.

Concentrators 14, 44 complete the physical connection between ports 16,so that an attached device 12 may communicate to other attached devices12. Concentrators 14, 44 are intelligent concentrators having controllogic and relay mechanisms to control the connection of the differentstations 12, the operation of which is well known to those skilled inthe art.

The main ring 26 connects the concentrator 14 with multipleconcentrators, such as the concentrator 44, that are serially connectedalong the ring network. The main ring may comprise a fiber optic cableor other type of known data transmission media, such as a shielded orunshielded twisted pair of copper wires. Each concentrator is connectedto the main ring 26 through a Ring-In/Ring-Out (RI/RO) device containedin the concentrator. The IN/OUT ports on the concentrators 14, 44connect together through main ring 26 to form a transmission circuit"ring", such that data travels in a clockwise direction around the ringnetwork. This enables stations inserted at a port 16 to communicate withnot only the other stations 12 attached to concentrator 14, but alsoother stations and servers on the network that are attached to adifferent concentrator, such as concentrator 44.

In the preferred embodiment, a token ring topology is used tointerconnect the attached devices or stations 12 within the LAN. Thetoken ring network allows unidirectional data transmission betweenstations in a ring-like circuit by a token passing procedure. The ringtopology permits tokens to be passed from a particular station 12 toanother station 12 attached to concentrator 14, or to another station 12attached to concentrator 44. A station 12 that is ready to send data cancapture the token and thereafter insert data for transmission over thenetwork.

The preferred token ring network is an IEEE 802.5 token-ring network,and particularly the IBM® Token-Ring network, which permitshigh-bandwidth peer-to-peer connectivity for the individual stations.The IBM® Token-Ring network may operate at a data rate of either 4Mb/sec or 16 Mb/sec, and supports as many as 260 stations per ring. TheIBM® Token-Ring network utilizes a differential Manchester code, whichis a digital encoding technique, wherein each bit period is divided intotwo complementary halves to encode base band digital waveforms. Atransition at the beginning of the bit period represents one of thebinary digit "0", while the absence of a transition at the beginning ofa bit period represents a binary digit "1". It is intended, and it willbe appreciated by those skilled in the art, that the present inventionis not limited in application to the preferred embodiment of a tokenring network, and that it may be utilized within any digital datatransmission or networking system.

An attached station or device 12 attempting to gain access to thenetwork has its network adapter begin its port insertion by entering a"Phase 0" of the adapter insertion process. During this phase, thenetwork adapter transmits flames to itself to determine if the lobe andthe transmit and receive circuitry are functioning properly. Therefore,during this phase, all frames are wrapped back to the adapter by theconcentrator so that the adapter will receive exactly what it sends outover the lobe if the link is good.

Once Phase 0 has completed, the network adapter enters "Phase 1" byapplying a phantom drive current to the lobe. The concentrator port 16detects the current's presence and sends a phantom detect interrupt tothe concentrator's CPU (not shown), which controls concentrator 14, 44.This phantom detect interrupt identifies the lobe attempting to insertitself into the network. The CPU, unless otherwise directed by thenetwork management system, engages a relay connected in series with theport 16 to connect the station attempting to insert with the network.Alternatively, some concentrators (e.g., the IBM® 8228 MAU) are notintelligent concentrators and may not have a CPU, or even be powered. Inthis type of concentrator, the phantom drive current charges acapacitor, which then flips a relay in the concentrator to insert theattached adapter.

LAN switches are known in the art of data communication and networkingand are used to provide data communication between devices or LANsegments attached to multiple ports of the LAN switch. A "LAN segment"may be defined as a group of nodes where all nodes utilize the same OpenSystem Interconnection (OSI) model physical layer. To connect twodevices or LAN segments, the LAN switch enables a node in one LANsegment to communicate with a node in a different LAN segment.Typically, the LAN switch receives data from a node in one LAN segmentand passes such data to another LAN segment which contains a destinationnode. The present invention is embodied in a multi-port LAN switch, suchas LAN switch 40, in a preferred embodiment. An example of a currentlyavailable LAN switch into which the present invention may beincorporated is the IBM® 8272 Token Ring Switch. While the presentinvention is described as being incorporated in a LAN switch, it will beappreciated by those skilled in the art that this invention applies toother types of data transfer units, including a "bridge", a "router" ora "gateway".

A LAN switch port performs several functions. Namely, a LAN switch portprovides the Media Access Control (MAC) and Physical Layer (PHY)necessary to couple to and communicate with a device attached to theport. Additionally, the LAN switch port maintains current portstatistics, including the number of good and bad frames passing throughthe port and the operational status of the port. "Bad" frames are thoseframes that contain errors. The port also maintains address tables thatlist the address of nodes connected to other ports of the multi-port LANswitch. Along with maintaining such address tables, the port alsoincludes circuitry for determining and selecting a destination port. Theport also includes buffers for buffering input and/or output frames.Buffering may be needed when a destination port is "busy" or when framesarrive at an aggregate rate that exceeds the capacity of the targetport. Finally, a LAN switch port provides interface logic to the "switchfabric". The switch fabric refers to the circuitry that carries datafrom one port to another. Such switch fabric may be a high-speed bus ora cross-bar switch.

Network adapter 42 is connected to port a of LAN switch 40 and networkadapter 43 is connected to port b of LAN switch 40. MAU 44 is connectedto port c of LAN switch 40 via lobe 32. Also, the two LAN switches 40and 46 are connected from port d of LAN switch 40 to port e of LANswitch 46. LAN switch 40 is also connected from its port e to port e ofLAN switch 47. Each of the above connections of multi-port LAN switch 40to the concentrator, network adapters, and switches is made by astandard cable having the standard two twisted pair cabling, as isrequired to conform with IEEE 802.5.

LAN switches 40, 46 and 47 operate as high speed communication bridgesas is known in the an of data communications and networking. A LANswitch can interconnect any two devices attached to its ports, therebyproviding a communications link between the devices. The LAN switch caninterconnect an attached adapter with another attached adapter, anattached adapter with an attached concentrator, or an attached adapterwith an attached LAN switch. The LAN switch also provides aninterconnecting link between two attached concentrators or two attachedLAN switches. The LAN switch enables connection of local loops,channels, or rings by matching circuits and facilitating accurate datatransmission.

According to the present invention, each port of the LAN switch isconfigured into the proper operational mode, either a port mode or anadapter mode. In the "port mode", the LAN switch is configured toproperly transmit and receive data and sink phantom drive current. Inthe "adapter mode", the LAN switch port is configured to properlytransmit and receive data and source a phantom drive current. Forexample, as shown in FIG. 1, ports a, b of LAN switch 40 would beconfigured in the port mode to receive and transmit data from networkadapters 42 and 43, respectively. Port e of LAN switch 40 would beconfigured in the adapter mode to enable LAN switch 40 to transmit andreceive data to MAU 44.

For example, when inserting onto the token-ring network, it must appearto network adapter 42 as if it were connected to a port of a MAU. Thus,ports a,b of LAN switch 40 must be configured to appear identical to aport of a MAU. Similarly, it must appear to MAU 44 that it is directlyconnected to an adapter. Consequently, ports c of LAN switch 40 must beconfigured to emulate to an adapter. According to the present invention,when network adapter 42 attempts to insert into the token-ring andinitiates a communication, LAN switch 40 provides a sink for the phantomdrive current. Therefore, according to a preferred embodiment of thepresent invention, in order to allow network adapter 42 to communicatewith network adapter 36, one LAN switch port would be placed in adaptermode and the other would be placed in port mode, enabling thetransmission of data between port d of LAN switch 40 and port e of LANswitch 46.

Referring now to FIG. 2, there is shown a schematic diagram of a singleport of the multi-port LAN switch of the present invention. As shown inFIG. 2, the port is configured in an adapter mode, in accordance with apreferred embodiment of the present invention. When an IEEE 802.5standard cable connects a device to the port, a first port connection 50is connected to the black/orange twisted-pair, and a second portconnection 52 is connected to the red/green twisted-pair. The first portconnection 50 is connected to a first winding of transformer 58, and thesecond port connection 52 is connected to a first winding of a secondtransformer 60. A second winding of first transformer 58 and a secondwinding of second transformer 60 are connected to relay 62. Bothtransmitter circuit 54 and receiver circuit 56 are also connected torelay 62.

The port includes transmission circuitry 54 and receive circuitry 56,which perform the transmitter and receiver functions, respectfully,required to support connectivity and communication with each possibleattached device, including network adapters, MAUs, and other LANswitches. Relay 62 creates electrical connection between circuits 54 and56, and transformers 58 and 60, as a function of "relay control 3"."Relay control 3" sets relay 62 to the connections shown in FIG. 2 whenthe port is in adapter mode. When "relay control 3" indicates adaptermode, relay 62 connects transmitter circuit 54 to first transformer 58,and connects receiver circuit 56 to second transformer 60. When "relaycontrol 3" indicates port mode, relay 62 electrically connectstransmitter circuit 54 to second transformer 60, and connects receivercircuit 56 to first transformer 58, as shown in FIG. 3.

Referring back to FIG. 2, a DC phantom source (not shown) is connectedto switches 64, which are controlled by "relay control 1". Switches 64connect the two poles of the DC phantom source to the first winding offirst transformer 58 to enable phantom drive current to be sourced fromthe port, out the first port connection, to the attached device on theblack/orange twisted-pair. Additionally, when a port is configured inadapter mode, it must provide a return-path for the phantom drivecurrent over the second port connection 52. Switches 66 are controlledby "relay control 2", and are switched to connect the first winding ofsecond transformer 60 to ground, when the port is in adapter mode.

In an alternative preferred embodiment of the present invention, anopto-coupler 69 is connected in series with the return-path. The outputof opto-coupler 69 provides an indication of whether the port is in theadapter mode or port mode (by the presence or absence of phantom drive).When configured in the port mode, the opto-coupler 69 will indicate thatcurrent is flowing through the return-path.

Referring now to FIG. 3, there is depicted a port of a multi-port LANswitch of the present invention configured in a port mode, in accordancewith the preferred embodiment of the present invention. As previouslydescribed, relay 62 is set in a cross-over configuration by "relaycontrol 3" in the port mode to connect transmitter circuit 54 withsecond transformer 60 and to connect receiver circuit 56 to firsttransformer 58. "Relay control 1" has opened switches 64 to disconnectthe DC phantom source from the first port connection 50, so that phantomdrive current is not sourced while the port is in port mode. Also,switches 66 have been switched by "relay control 2" to connect the firstwindings of transformers 58 and 60 (through resisters 68), and toprovide a return-path (i.e. ground) for phantom drive current sourced bythe attached device on the black/orange twisted-pair wires.

As can be seen from the above description, the LAN switch of the presentinvention is capable of emulating either a network adapter or aconcentrator port at each of its own ports. When emulating a networkadapter, the LAN switch port will source phantom drive current andtransmit data over the black/orange twisted-pair wires and will receivedata and provide a ground for the phantom drive current on the red/greentwisted-pair. When emulating a concentrator port, the LAN switch portwill receive data over the black/orange twisted-pair wires and transmitdata over the red/green twisted-pair wires. In addition, while in thisport mode, the LAN switch port provides a DC electrical connectionbetween the first and second port connections to provide a return-pathover the red/green twisted-pair wires for the phantom current source.

Referring now to FIGS. 4-6, there is depicted three schematic blockdiagrams, each showing a port of the LAN switch of the present inventionin either the adapter mode or port mode, as is appropriate for theattached device. FIG. 4 shows a standard adapter connected to a port ofthe multi-port LAN switch of the present invention configured in portmode, in accordance with the preferred embodiment of the presentinvention. The standard adapter includes transmitter circuit 70 andreceiver circuit 72, and transformer 74 and 76. Since the LAN switchport must be in port mode to communicate with a standard adapter, relay62 is properly "crossed-over" to connect transmitter circuit 70 toreceiver circuit 56, and to connect transmitter circuit 54 with receivercircuit 72. Also, switches 66 are switched to connect the first windingsof the port transformers 58 and 60 to provide a DC return-path for thephantom drivecurrent.

FIG. 5 depicts the LAN switch port of the present invention configuredin adapter mode for connection with a standard MAU, in accordance with apreferred embodiment of the present invention. In adapter mode, switches64 are closed to provide a phantom drive current to the standard MAUover the black/orange twisted-pair wires. The MAU returns the phantomdrive current over the red/green twisted-pair wires to ground throughswitches 66.

FIG. 6 depicts a schematic block diagram showing a port of a first LANswitch of the present invention in the adapter mode, and a port of asecond LAN switch of the present invention in the port mode, inaccordance with a preferred embodiment of the present invention. Here,the two LAN switches of the present invention are connected using astandard cable, rather than a "crossed" cable, as would be required byprior art LAN switches.

As is known by those skilled in the art, a LAN switch is an intelligentdevice having data processing capabilities, including determining thedestination node of a frame of data, based on addressing informationreceived with the data, and transferring the received data to the portconnecting with the destination node. As is described hereinbelow, theLAN switch of the present invention automatically determines what typeof device is attached to each of its ports, and then configures thoseports in either the port mode or the adapter mode to allow propercommunication between each of the attached devices. The advantages ofthis capability are: 1) All cabling can be of same polarity (i.e., nocrossed cables are needed), and 2) no manual intervention is required(e.g., installation of special cables or setting of port configurationswitches). The overall advantage is lower-cost and higher-reliabilityinstallation and maintenance.

Thus, an additional feature of the LAN switch port of the presentinvention is its capability of automatically determining the type ofdevice connected to the port, and configuring the port to the properoperational mode required to enable communication with the device. Forexample, a LAN switch-to-LAN switch connection requires oppositetransmit and receive polarities between the two switch ports (i.e., onebeing port mode and one being adapter mode). The method describedhereinbelow senses the attached LAN switch and properly configures theport to allow communication with the attached LAN switch.

The port also has the capability of determining whether an attachedadapter or LAN switch is a half-duplex or a full-duplex device. Forexample, as seen in FIG. 1, adapter 42 is a half-duplex (HDX) adapterand adapter 43 is a full-duplex (FDX) adapter. Also, LAN switch 47 is aFDX switch, and LAN switch 46 is a HDX switch. In token-ringnomenclature, half-duplex (HDX) refers to the normal token passingaccess protocol as defined by IEEE 802.5. Full-duplex (FDX) refers tothe transmit immediate access protocol currently being defined by IEEE802.5. FDX operation is based on a point-to-point connection of twodevices and does not use tokens. In the FDX mode, each device cantransmit and receive at any time (i.e., without waiting for a token). Amethod of FDX operation for a token-ring network is described in"Token-Ring 16/4 Adapter with Full Duplex Switching Mode.", Strole, N.,Christensen, K., Noel, F., and Zeisz, R., IBM Technical DisclosureBulletin, Vol. 37, No. 04A, pp. 617-618, April 1994.

Each LAN switch port contains a Token Ring Controller (not shown),including full Media Access Control (MAC) and Physical (PHY) layerimplementations. In addition, each Token Ring Controller includes acentral processing unit (CPU), called the port CPU (not shown). The portCPU controls the determination of connection type and then performs theinsertion of the attached device by configuring the LAN switch port intothe proper operational mode. The LAN switch port is configured byasserting one or more mode signals, in particular, the relay controlsignals 1-3, as was described hereinabove for configuring into eitherthe adapter mode or port mode. This process is described in detail inconjunction with FIGS. 7-8 below. Thus, there are two possibleconfigurations for a port in adapter mode (FDX adapter or HDX adapter)and two possible configurations for a port in port mode (FDX port or HDXport).

Referring now to FIGS. 7-8, there is depicted a flow diagram of themethod of automatically determining an attached device type and forconfiguring a LAN switch port for the multi-port LAN switch of thepresent invention, in accordance with a preferred embodiment of thepresent invention. Referring first to FIG. 7, the process begins at step100 where the port CPU starts the timer T(passive). This timer times apassive detect mode. The total time of T(passive) is determined via arandom number generator and is in the range from 3 seconds to 3.2seconds. Using a random number generator insures that two connected LANswitch ports cannot both always be in the passive detect mode at thesame time, and, consequently, one will attempt to insert in the other.Also, at step 100, the switch port is set in the port mode.

At step 105, a determination whether T(passive) has expired is made. IfT(passive) has expired, the process proceeds to step 145 (see FIG. 8),as indicated by A. If T(passive) has not expired, the process proceedsto decision block 110, where it is determined whether a FDX frame hasbeen received. If so, the LAN switch port is opened (i.e., configured)as a FDX port, as shown at step 115. If a FDX frame has not beenreceived at step 110, the LAN switch port determines whether a phantomdrive current has been asserted by an attached device, as shown at step120. If the phantom drive current is not detected at step 120, theprocess returns to step 105. The phantom drive current can be detectedby a sensor such as the opto-coupler 69 shown in FIG. 2. If either ofthe decisions at steps 110 or 120 are affirmative, the attached deviceis attempting to insert into the port.

If phantom drive current is detected at step 120, the port hasdetermined that an adapter or a LAN switch port in adapter mode isattached to the port. The process proceeds to step 125, where the porttransmits a FDX frame from transmit circuitry 54 over the red/greentwisted-pair to the attached device's receiver circuitry. Also, at step125, a timer T(resp) is started. The timer T(resp) provides a period inwhich the LAN switch port waits for a response transmission of a FDXregistration frame. The value of T(resp) is 800 milliseconds in apreferred embodiment of the present invention.

The process then proceeds to step 130 where the port listens for an FDXframe to be transmitted by the attached adapter or switch. If a FDXframe is received, the LAN switch port is opened (i.e., configured) as aFDX port, as shown at step 115. If a FDX frame has not been received atstep 130, and the timer T(resp) has not expired, as determined at step135, the process loops back to step 130 where the port continues tolisten for an FDX frame to be transmitted by the attached adapter. Ifthe timer T(resp) has expired, the process proceeds to step 140 wherethe LAN switch port is opened (i.e., configured) as a HDX port. Also atstep 140, the port transmits a ring purge frame. The purge frame is astandard IEEE 802.5 defined MAC frame used to clear a ring segment offrames or tokens.

Referring now to FIG. 8, there is depicted a flow diagram, continuingthe flow diagram of FIG. 7, showing the method of automaticallydetermining the connection type of an attached device and configuringthe LAN switch port according to a preferred embodiment of the presentinvention. When the process proceeds to step 145 (as indicated by A) asa result of the timer T(passive) expiring (determined at step 105), awrap timer T(wrap) is started. The value of T(wrap) is 30 millisecondsin a preferred embodiment. Also, at step 145, the relay 62 in the LANswitch port is set to the uncrossed position, or adapter mode, and aDuplicate Address Test (DAT) MAC frame, as defined in the IEEE 802.5standard, is transmitted by the port.

The port then listens for any type of frame to be received by itsreceiver circuitry, at step 150. If no frame has been received, the portcontinues to loop back to step 150 until the timer T(wrap) has expiredat step 155, and the process returns to step 100 (as indicated by B). Ifa frame has been received by the port prior to the timer T(wrap)expiring, the process proceeds to step 160, where a fault timer T(fault)is started (the value of T(fault) is 10 to 15 seconds in a preferredembodiment) and a phantom drive current is sourced by the port closingswitches 64!, thereby asserting a phantom drive current on theblack/orange twisted-pair. If the timer T(fault) has not expired, asdetermined at step 165, the port determines whether any type of framehas been received, at step 170. If no frame has been received, theprocess returns to step 165. If a frame is received, the port determineswhether the frame is a FDX frame, at step 175. If the frame is a FDXframe, the port sets the phantom drive current off, and opens (i.e.,configures) in the FDX adapter mode, as indicated at step 180. If it isdetermined that the frame is not a FDX frame at step 175, then theattached port must be an HDX port. In that case, the LAN switch portsets the phantom drive current off and opens (i.e., configures) as a HDXadapter, as indicated at step 185.

If the fault timer T(fault) expires, as determined at step 165, prior tothe port receiving any frame, the process proceeds to step 190, where itis determined whether a wiring fault condition has been detected. If awiring fault condition is present (for example, when a cable is notconnected to the port), the phantom drive current will not have areturn-path and a fault will be detected. When a fault is detected atstep 190, the process returns to step 100 (as indicated by B). If nofault is detected at step 190, the process proceeds to step 185, wherethe phantom drive current is turned off and the port is opened (i.e.,configured) as a HDX adapter.

In summary, the LAN switch of the present invention is capable ofemulating either a network adapter or a concentrator port at each of itsown ports. When emulating a network adapter, the LAN switch port willsource phantom drive current and transmit data over the black/orangetwisted-pair wires and will receive data and provide a ground for thephantom drive current on the red/green twisted-pair. When emulating aconcentrator port, the LAN switch port will receive data over theblack/orange twisted-pair wires and transmit data over the red/greentwisted-pair wires. In addition, while in this port mode, the LAN switchport provides a DC electrical connection between the first and secondport connections to provide a return-path over the red/greentwisted-pair wires for the phantom current source.

While the invention has been particularly shown and described withreference to a preferred embodiment, it will be understood by thoseskilled in the art that various changes in form and detail may be madetherein without departing from the spirit and scope of the invention.

What is claimed is:
 1. A data transfer unit for a network, each port ofthe data transfer unit having a first port connection, a second portconnection, a transmitter circuit and a receiver circuit, each port ofthe data transfer unit comprising:a first transformer having a firstwinding connected to the first port connection, and a second winding; asecond transformer having a first winding connected to the second portconnection, and a second winding; a switching system that switches theport to a first mode or a second mode as a function of a mode signal forthat port, wherein the switching system in the first mode connects thesecond winding of the first transformer to the transmitter circuit andthe second winding of the second transformer to the receiver circuit,and wherein the switching system in the second mode connects the secondwinding of the first transformer to the receiver circuit and the secondwinding of the second transformer to the transmitter circuit.
 2. A datatransfer unit for a network according to claim 1, each port furthercomprising:a first switch; a DC voltage source switchably connected tothe first port connection through the first switch; and a second switchhaving a first and second position, and connecting the second portconnection to a DC return-path in the first position and to the firstport connection in the second position.
 3. A data transfer unit for anetwork according to claim 1, wherein every transmitter circuit isinterconnected with every receiver circuit such that each port cancommunicate with every other port.
 4. A data transfer unit for a networkaccording to claim 1, wherein a port is connected to a data transferunit.
 5. A data transfer unit for a network according to claim 1,wherein a port is connected to a standard adapter.
 6. A data transferunit for a network according to claim 1, wherein a port is connected toa concentrator.
 7. A data transfer unit for a network unit according toclaim 1, wherein the data transfer unit is a multi-port LAN switch.
 8. Adata transfer unit for a network, each port of the data transfer unithaving a second port connection, a first port connection, a transmittercircuit and a receiver circuit, each port of the data transfer unitcomprising:a switching system that switches the port to a first mode ora second mode as a function of a mode signal for that port, wherein theswitching system switched to the first mode connects the first portconnection to the transmitter circuit and the second port connection tothe receiver circuit, and wherein the switching system switched to thesecond mode connects the first port connection to the receiver circuitand the second port connection to the transmitter circuit; a firstswitch; a DC voltage source switchably connected to the first portconnection through the first switch; and a second switch having a firstand second position, and connecting the second port connection to a DCreturn-path in the first position and to the first port connection inthe second position.
 9. A data transfer unit for a network according toclaim 8, wherein every transmitter circuit is interconnected with everyreceiver circuit such that each port can communicate with every otherport.
 10. A data transfer unit for a network according to claim 8,wherein a port is connected to a data transfer unit.
 11. A data transferunit for a network according to claim 8, wherein a port is connected toa standard adapter.
 12. A data transfer unit for a network according toclaim 8, wherein a port is connected to a concentrator.
 13. A datatransfer unit for a network according to claim 8, wherein the datatransfer unit is a multi-port LAN switch.
 14. A data communicationsnetworking system comprising:a plurality of network adapters; aconcentrator connecting one or more of the plurality of network adaptersto form a token-ting network; and a first data transfer unit having oneof the plurality of network adapters connected to a first port of thedata transfer unit and the concentrator connected to a second port ofthe data transfer unit, each port of the first data transfer unit havinga first port connection, a second port connection, a transmitter circuitand a receiver circuit, and further each port of the data transfer unitincluding:a switching system that switches the port to a first mode or asecond mode as a function of a mode signal for that port, wherein theswitching system switched to the first mode connects the first portconnection to the transmitter circuit and the second port connection tothe receiver circuit, and wherein the switching system switched to thesecond mode connects the first port connection to the receiver circuitand the second port connection to the transmitter circuit; a firstswitch; a DC voltage source switchably connected to the first portconnection through the first switch; and a second switch having a firstand second position, and connecting the second port connection to a DCreturn-path in the first position and to the first port connection inthe second position.
 15. A data communications networking systemaccording to claim 14, wherein every transmitter circuit isinterconnected with every receiver circuit such that each port cancommunicate with every other port.
 16. A data communications networkingsystems according to claim 14, wherein a third port of the data transferunit is connected to a first port of a second data transfer unit.
 17. Adata communications networking system according to claim 14, wherein afirst mode signal switches the switching system of the first port of thefirst data transfer unit to the second mode.
 18. A data communicationsnetworking system according to claim 14, wherein a second mode signalswitches the switching system of the second port of the first datatransfer unit to the first mode.
 19. A data communications networkingsystem according to claim 14, wherein the first data transfer unit is aLAN switch.